The invention concerns a method of controlling welding conditions of a resistance welder, in which the nugget generating state can be estimated with general-purpose properties, and in which accurate and high-quality resultant welding can be obtained. Through a process of using a heat conduction simulator for performing simulation of a welding portion on the basis of heat conduction calculation, estimating the status quantity expressing the state in which a nugget is formed in the welding portion during welding, comparing the status quantity with reference status quantity led out of a physical phenomenon generating during welding at the time of estimation, and correcting the heat conduction simulator on the basis of the comparison result, it is possible to control the welding conditions accurately, and it is possible to obtain accurate and high-quality resultant welding.